FIG. 1 illustrates a block diagram of an integrated drive generator 10 of a type manufactured by the assignee of the present invention for generating three phase 400 Hz 120 volt alternating current. The integrated drive generator 10 is driven by a power takeoff 12 from an airframe propulsion engine which varies in speed during operation of the airframe. The power takeoff 12 is coupled to a constant speed drive transmission 14 which functions to produce a constant speed output on shaft 16 while the rotational speed of the power takeoff varies. It should be understood that the construction of the constant speed drive transmission 14 is conventional. Furthermore the connection of the constant speed drive transmission to the shaft 16 is illustrated only schematically. The integrated drive generator 10 has a permanent magnet generator 18, wound field exciter 20 and main generator 22, each of conventional construction, having rotors mounted on shaft 16 which is supported by bearings (not illustrated) which are mounted in a housing (not illustrated) of the integrated drive generator. The permanent magnet generator 18 has a permanent magnet rotor 24 mounted on the shaft 16. The stator 26 of the permanent magnet generator 18 outputs alternating current which is rectified by rectifier 28 to produce field excitation current which is applied to the stator 30 of the wound field exciter 20. The rotor 32 of the wound field exciter, mounted on shaft 16, outputs alternating current which is rectified by rectifier 34. Rectified current from the rectifier 34 is applied to the field windings of the rotor 36 of the main generator 22. The stator 38 outputs three phase 400 Hz 120 volt alternating current for use in powering the various electrical loads on the airframe.
The weight and size of an integrated drive generator is of extreme importance in the design of airframes. Unnecessary weight lessens the overall efficiency of the airframe and its load carrying capability. Increased size in an integrated drive generator can interfere with the mounting of the integrated drive generator on the propulsion engine as a consequence of interference between the integrated drive generator and the cowling of the engine. Shortening of the overall length of the housing of the integrated drive generator with respect to the length of the drive shaft 16 is important in reducing weight, facilitating mounting of the integrated drive generator with respect to the engine cowling and reducing overhung moment which lessens the requirement for reinforcing of the mounting flange on the engine where the integrated drive generator is attached.
A four pole main generator 22 was utilized as a consequence of the rotor of a four pole main generator being shorter than a two pole generator of comparable power output, in order to shorten the length of the casing to reduce overhung moment. However, a four pole main generator 22 has a rotor 36 which has a larger diameter than a rotor of a two pole main generator of comparable power, which adds overall weight to the main generator over the overall weight of a two pole main generator of comparable power output. A two pole rotor 36 requires a longer rotor than the rotor of a corresponding four pole rotor, to produce the same amount of energy. The increased diameter of the shaft 16 necessary to support the rotors of the permanent magnet generator 18, wound field exciter 20 and main generator 22 increased the weight of the shaft with a concomitant overhung moment penalty and presented more difficult spatial requirements of mounting the integrated drive generator with respect to the cowling of the airframe propulsion engine.